DOUBLE JEOPARDY: The Vulnerability Spiral of Compound Climate Risk Across 5 Islands

SAKSHI D. MASKE 

Low islands face two threats at once: the sea rising, and the reefs and mangroves that buffer it weakening. I measured both across 5 island groups in 3 ocean basins (Maldives, Lakshadweep, Seychelles, Fiji and the Canary Islands), testing mangroves and coral reefs as separate pathways. Exposure is the share of people (WorldPop 2020) living on land at or below 1 m, on a bare earth elevation model (DeltaDTM v1.1), with the Copernicus GLO-30 surface model and the share of OpenStreetMap named places as comparisons. On bare earth, 11.8% of people in the Maldives and 6.4% in Lakshadweep live at or below 1 m, against 8.9% and 0.8% on the surface model; counting named places instead puts Seychelles (24.7%) level with the Maldives (28.8%). Mean Degree Heating Weeks over mapped reef were higher in 2016–2020 than in 1996–2000 on all 4 reef islands, most in Seychelles (+0.52 °C-weeks), but no monotonic trend is significant once serial correlation is accounted for (corrected seasonal Mann-Kendall p = 0.14 to 0.90). Mangrove extent (Global Mangrove Watch, 1996, 2010, 2020) shows no decline. A compound score of exposure and reef heat stress puts Seychelles (0.58) and the Maldives (0.56) in a near tie, with the top island changing at an exposure weight of 52%. Protection of the 10 km coastal zone ranges from 1.3% (Maldives) to 71.4% (Seychelles) and does not follow this score (r = −0.10, n = 4, descriptive only). A building-level extension adds a policy question: whether statutory coastal setbacks are drawn where low-lying buildings are. Of Overture Maps buildings at or below 1 m, 92–93% lie landward of the 25 m setback in Seychelles and the 20 m zone in Lakshadweep, the Maldives and Fiji have no setback for inhabited land, and only the Canary Islands' 100 m Ley de Costas line covers most of them (85%); on the four tropical island groups, half of the low buildings stand 80–136 m from the shoreline. The main result is about method: the elevation model, the exposure measure and the weighting each change which island looks most vulnerable, and an index used to allocate adaptation resources should report all three.